After an introduction to the contents of the book which examines the use of the Cornish language in the nineteenth century, this chapter introduces the terminology used to discuss language decline and describe the status of endangered languages. Following this, frameworks which have been developed to gauge a language’s vitality are introduced, including a discussion of the reasons behind the decision to use Lee and van Way’s Language Endangerment Index (LEI) to examine the status of Cornish in the nineteenth century. The focus then turns to an examination of the previous literature on the history of the Cornish language and how these texts have addressed the issue of the status of Cornish in the nineteenth century or otherwise. Finally, the chapter discusses questions surrounding the use of historical sources, including newspapers, letters, and novels in this research.